Sensor for measuring strain

ABSTRACT

A tension sensor including an optical fibre having a Bragg grating is disclosed, the sensor including a housing and a support member shorter that the housing coaxially disposed therein and connected to the housing at one end. The fiber runs along the longitudinal axis of the housing and is connected to one end of the housing and to at least one location on the support member. A second Bragg grating may be provided within the support member to allow for additional measurements for temperature compensation purposes.

FIELD OF THE INVENTION

The present invention relates to a device for measuring tensile forcesas defined in the introductory part of claim 1.

BACKGROUND OF THE INVENTION

The present invention is based on the principle of utilizing afibreoptical Bragg grating. A Bragg grating is single modus fibre withpermanent periodic variation of the refractive index over a fibre lengthof, for example 0.1 to 10 cm. The variation in the refractive index isestablished by illuminating the fibre with a UV laser. A Bragg gratingreflects light with a wavelength that depends upon the refractive indexand the space related period of the variation of the refractive index(the grating period), while light beyond this wavelength will passthrough the grating more or less unhindered. The light reflected by theBragg grating will exhibit a wavelength that varies as a function of ameasurable quantity that changes the refractive index of the fibrematerial grating and/or the fibre length in the gratina zone (gratingperiod). Tension in the fibre or temperature variations will thereforelead to a change of the wavelength of the light reflected by the Bragggrating.

For practical purposes one can, for example measure the temperature inthe region −100° C to +250° C with (in the order of) 20 different pointsalong the fibre for fibres with a length of up to 50-100 km. Usingvarious multiplexing techniques, the number of measurement points can beincreased. Examples of areas of application are temperature surveillanceof power cables, pipelines, electrical transformers, engines andtemperature monitoring of industrial processes.

A number of devices for measurement of tension in mechanicalconstructions exist. For special purposes where there is little spaceavailable, high temperature. high tension and so forth, all knowndevices for measurement of tensile forces have functional disadvantages.For example present measurement of tension under water is made withtensile sensitive sensors based on electrical elements, which in suchenvironments exhibit low reliability. For other areas of applicationthere may be little space available for installing extra components,such as tension sensors based on electrical induction or capacity(typical diameter 10-20 mm). Another example is the surveillance of darnwith sensors based on electrical strain gauges. In such connectionslightening strikes have sometimes rendered the sensor elements or theelectronic circuits passive, and thus disabled the tension surveillance.

Accordingly there is a need for a tension sensor with mainly passivecomponents that can be utilized in difficult environments and narrowspaces.

The objective of the present invention is to provide a device of thistype for tension measurement in and on mechanical constructions.

SUMMERY OF THE INVENTION

This objective is achieved with a device according to the characterizingpart of claim 1. Beneficial features are disclosed by the dependentclaims.

The invention relates to a device for measuring tension in mechanicalconstructions, the device comprising:

an optical fibre provided with a first Bragg grating,

an elongated housing arranged to encompass the optical fibre and to beattached to the construction to be measured, whereby the housingincludes a first end and a second end and includes a first attachmentsite at the first end of the housing in order to establish a solidattachment between the housing and the optical fibre,

an elongated support member with a mechanical strength greater than thestrength of the optical fibre and with a length shorter than the lengthof the housing. whereby one end or section of the support member issolidly attached to the housing at the second end of the same, and asecond end extending freely along a part of the length of the housing.the support member exhibiting a second attachment site in order toestablish a solid attachment between the support member and the opticalfibre,

thus establishing a segment of an optical fibre comprising said Braoggrating strapped between said first and said second attachment site ofthe housing and the elongated support member respectively .

This principal design of a tension sensor renders it possible to producetension sensors with very small dimensions and with a measurement rangefrom low tensions to tensions of several thousand microstrain in distantpositions. The device also has the possibility of measuring tension indifferent positions along the same optical fibre.

Examples of mechanical constructions is meant constructions which canbenefit from the invention are bridges, dams, platforms. cables,flexible pipes and the like.

To compensate for temperature related variations in measurementsdetected by the first Bragg grating, the support member preferentiallyincludes a third attachment site for an optical fibre, localized in theregion between the second point of attachment and the holding member ofthe section of the support member that extends freely along the housing,whereas the optical fibre exhibits a second Bragg-grating (referencegrating) localized between the second and the third attachment sites.Since the reference grating is arranged in the part of the supportmember which is free in relation to the housing, there will only beminor strain on it from mechanical strain that is exerted to thehousing, so that variations in measurements conducted by the referencegrating mainly relate to temperature variations.

In a preferred embodiment of the invention the housing has a generallycylindrical shape and a generally cylindrical bore, the support memberis a generally cylindrical shaped tubing with an external diameter lessthan the internal diameter of the housing.

The support member is preferably constructed from a material with thesame thermal expansion coefficient as the surrounding construction to bemeasured, thus compensating tension loads which are merely temperaturerelated.

BRIEF DESCRIPTION OF THE DRAWING

The invention is described in further detail by reference to a preferredembodiment illustrated by the accompanying drawings, where

FIG. 1 shows an axial crossection schematic of an example of the deviceaccording to the invention for monitoring tension in mechanicalstructures, and

FIG. 2 shows an example of the practical utilization of the principle ofthe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a first embodiment of a tension sensor according to theinvention. The tension sensor includes an elongated generallycylindrical housing 101 with an inner cylindrical bore 102 and with afirst end 101 a and a second end 101 b, arranged to encompass an opticalfibre 120 extending through the bore 102 of the housing.

Inside the bore 102, at the second end 101 b of the housing is arrangeda support member in the form of a generally cylindrical tube 103 with adiameter less than the internal diameter of the housing 101 and a lengthshorter than the length of the housing 101. The internal tube 103extends generally coaxially with the housing 101 and has an internalopening with dimension sufficient to encompass optical fibre 120. Thetube 103 is in its one end solidly attached to the second end 101 b ofthe cylindrical housing 101 by means of a holding member 110, such as aglue joint, and extends freely into the cylindrical bore 102 of thehousing 101 thus establishing a ring shaped room 109 between theexternal surface of the tube 103 and the internal surface of the housing101. The free part of the tube 103 is thus not exposed to tension forcesthat is exerted to the housing 101.

At the first end of the housing 101 a is arranged a first point ofattachment 111 for the optical fibre 120 to the housing 101. In thisembodiment the first point of attachment 111 also constitutes a sealingor a barrier against intrusion of pressure or fluids from thesurroundings, and may be established, for example, in the form of aglued joint arranged sealingly in the space between the fibre 120 andthe internal surface of the housing 101.

The fibre includes a first Bragg grating 121 freely strapped between thefirst point of attachment 111 and a second point of attachment 112localized at the free end of the internal tube 103, thus providing astrapped fibre segment between the housing 101 by the point ofattachment 111 and the internal tube 103 by the second point ofattachment 112.

A second Bragg grating 122 is strapped between the second attachmentsite 112 and a third attachment site localized to the part of the tubing103 which is not exerted to tension forces that effect the housing 101,i.e. in a distance from the holding member 110.

The housing 101 to be exposed to tension is externally attached to thesurrounding mechanical construction to be monitored along the entirelength of the housing (101) (e.g. by gluing) or at both ends of thehousing.

When the housing 101 is stretched the first Bragg grating 121 willundergo the same total elongation as the housing when the tension istransferred by the tubing 103. The second Bragg grating 122 and thetubing 103 will experience an elongation that is substantially less asthe mechanical strength (crossection and elasticity modulus) of the tube103 is substantially greater than that of the fibre.

The optical fibre 120 is made of glass with a small diameter and must beprotected against the surroundings in most of the practical applicationsin the embodiment shown in FIG. 2, which shows an alternative embodimentin the form of an elongated tube, separate tubes 130 and 131 extendpartially into the bore 102 of the housing 101 a nd are solidly attachedto the housing 101. This embodiment provides extra protection and may beutilized on both sides of the housing 101. The tubes 130 and 131 areattached to the housing 101 so that each end may constitute or be a partof the attachment site 111 and the holding member 110 respectively, asshown in FIG. 2. The Bragg gratings are for the sake of clarity notshown here.

If the housing 101, which is exposed to tension, is attached at each endof the housing 101, e.g. at the holding members 111 and 110, and isexerted to a force F (not illustrated), the elongation ε_(H) may beexpressed by the following equation:$\varepsilon_{H} = {\frac{\Delta \quad H}{H} = \frac{F}{E_{H} + A_{H}}}$

here delta ΔH is the absolute change in length of the housing 101, H isthe starting length of the housing 101 (alternatively between theattachment sites (not shown) on the housing to the construction to bemeasured), A_(H), is the crossectional area of the walls of the housing101 a nd E₁₁ is the elasticity modulus for the material of the housing.As made evident from the equation above. the specific extensioncompression of the housing will increase with increasing force and withdecreasing strength (crossectional area and elasticity modulus) of thehousing.

If the housing 101 is solidly attached to the surrounding constructionalong its entire length H. e.g. by gluing or welding, the relativeextension ε_(H) of the housing will be the same as for the surroundingconstruction.

Since the real extension ΔH of the housing 101 is the same as theextension of the fibre with the Bragg grating 121 between the attachmentsites 111 and 112, the specific extension ε₁ of the grating 121 may beexpressed by the following equation:$\varepsilon_{1} = {{\frac{H}{L_{1}} \cdot \varepsilon_{H}} = {\frac{H}{H - R} \cdot \varepsilon_{H}}}$

where H is the length of the housing 101, L₁ is the length of the fibrewith the Bragg grating 121 (between attachment sites 111 and 112) andε_(H) is the specific extension of the housing 101 a s defined by theprevious equation. R is the length of the internal tube 103 between theattachment site 112 and the second holding member 110. From thisequation one can recognise that using a longer tube 103 as compared tothe grating length L₁ (R+L₁=H),will lead to an enhancement of thegrating extension ε₁ compared to the extension ε_(H) of the housing 101.

In one embodiment the entire device, with the obvious exception of thefibre, is constructed of metal. Metal materials are generally preferredas they combine high mechanical strength with weldability and a certaindegree of elasticity, which is required for applications for themeasurement tensile forces. For applications where small diameters ofthe sensors are required, both housing 101 , internal tube 103 andprotecting tube 130, 131 can be made from cannula tubes. The tensionsensors may thus be produced with diameters of the housing and tubes inthe order of 1 mm in diameter. For measuring forces the crossection ofthe housing exposed to strain and the mechanical properties of thematerial of the housing, is adjusted according to the force range to bemeasured.

To compensate temperature dependent wavelength displacement withrelation to light reflected from the Bragg grating, which primarily iscaused by a change in the refractive index of the fibre material as adirect function of the temperature change, a reference grating 122(Bragg grating) is established. As indicated above the reference gratingis arranged between the attachment sited 112 and 113 in the internaltube 103. Thus this grating, is only to a small extent exposed tomechanical forces caused by tensions on the housing 101 or forces thatpropagate along the fibre 120. The device is calibrated at differenttemperatures to achieve a measurement of wavelength displacement as afunction of tension which is as unaffected by temperature as possible.

If the internal tube 103 is manufactured from the same material as thesurrounding construction, the sensor may be calibrated to eliminateextension caused by thermal expansion of the surrounding structure. Thesensor will thus only respond to extensions caused by mechanicaltension.

From the foregoing description it is evident for a person skilled in theart that the different components do not necessarily have the geometryshowed by the drawings. Consequently, the member to be exposed totension may for example, have a crossection that deviates from acircular shape. it may be oval, square etc. The same applies to theother components. The central issue with the invention is, however, thatthe member to be exposed to tension shall be able to transmit extensionto the connective member and further to the connected Bragg, grating.

The invention thus provides a device for measuring tension in mechanicalconstructions. which enables measurement over a broad range of tensionsand with high precision and which compensates for deviations caused bytemperature fluctuations. In addition the device according to theinvention can be designed to be very small and can therefore beinstalled in places where measurement usually has not been possible.Another advantage with the device according to the invention is that thefibre is not exposed to external hydrostatic pressure, and willtherefore exhibit a high reliability. Finally, this design does notrequire pressure tight connections for the fibre.

What is claimed is:
 1. A tension measuring device, comprising: anoptical fibre having a first Bragg grating (121); an elongated housing(101) encompassing the optical fibre (120); the housing having a firstend (111 a ) and a second end (101 b ) and a first attachment site (111)located at the first end (111 a ) of the housing, said optical fibrebeing secured to said housing at said first attachment site (111); anelongated support member (103) having a mechanical strength greater thanthe strength of the optical fibre (120) and having a length shorter thanthe length of the housing (101) and being disposed within said housing,and encompassing said optical fibre, wherein one end or section of thesupport member (103) is solidly attached to the housing (101) at thehousing second end (101 b ), and a second end of the support memberextends freely within the housing (101), the support member (103)further including a second attachment site (112) for securing theoptical fibre (120) within said support member (103), wherein said firstBragg grating (121) is held between said housing first attachment site(111) and said support member (112). Wherein the tension in said housingis measured by measuring the wavelength of a reflected light by saidfrist Brag grating.
 2. Device according to claim 1, further comprising athird attachment site (113) for the optical fibre (120) located in aregion between the second attachment site (112) and a holding member(110) in the region where the support member (103) is free from contactwith the housing (101), and a second Bragg grating (122) located betweenthe second attachment site (112) and the third attachment site (113). 3.Device according to claim 1, characterized in that the housing (101) hasa generally cylindrical shape and with a generally cylindrical bore(102), and that the support member (103) is a generally cylindricaltubing with an external diameter less than the internal diameter of thehousing (101).
 4. Device according to claim 1, characterized in that thesupport member (103) is constructed from a material with the samethermal expansion coefficient as a surrounding construction to bemeasured.
 5. Device according to claim 1, further comprising a firsttube portion disposed between the interior of said housing and saidfirst attachment site (111), and a second tube portion disposed betweenthe interior of said housing and said holding member (110) at theopposite end of said housing, each of said first and said second tubeportions being sealingly and solidly secured to said portions of theinterior of the housing.
 6. Device according to claim 2, furthercomprising a first tube portion disposed between the interior of saidhousing and said first attachment site (111), and a second tube portiondisposed between the interior of said housing and said holding member(110) at the opposite end of said housing, each of said first and saidsecond tube portions being sealingly and solidly secured to saidportions of the interior of the housing.
 7. Device according to claim 3,further comprising a first tube portion disposed between the interior ofsaid housing and said first attachment site (111), and a second tubeportion disposed between the interior of said housing and said holdingmember (110) at the opposite end of said housing, each of said first andsaid second tube portions being sealingly and solidly secured to saidportions of the interior of the housing.
 8. Device according to claim 4,further comprising a first tube portion disposed between the interior ofsaid housing and said first attachment site (111), and a second tubeportion disposed between the interior of said housing and said holdingmember (110) at the opposite end of said housing, each of said first andsaid second tube portions being sealingly and solidly secured to saidportions of the interior of the housing.